A ball is dropped from rest from a height of 45 m. Ignoring air resistance and taking g = 10 m/s^2, what is the time taken for the ball to reach the ground?
Show the answer and explanation
Correct answer
B. 3.0 s
Principle or equation
For free fall from rest, the displacement is given by h = (1/2) g t^2.
Why this answer is correct
Using h = (1/2) g t^2, with h = 45 m and g = 10 m/s^2, we have 45 = 5 t^2, so t^2 = 9 and t = 3.0 s.
Example
A ball dropped from 20 m takes t = sqrt(2h/g) = sqrt(4) = 2 s.
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